1. Field of the Invention
The present invention relates to an ink jet printer capable of generating different ink droplets for one print period and an ink jet printing method for generating different ink droplets for one print period.
2. Discussion of the Related Art
Generally, the ink jet printer is provided with a print head having a number of nozzles arrayed in the slow scan (vertical) direction. To print on a printing material, e.g., a sheet of paper, the printer drives a carriage mechanism to move the print head in the main scan (horizontal) direction and feeds the paper in the horizontal direction, and repeats the operations. At the same time, the printer causes the print head to eject ink droplets from the nozzles at predetermined timings in accordance with dot pattern data generated by developing print data received from a host computer. The ejected ink droplets land on the paper to print an image represented by the print data on the paper. To print, a control as to whether the ink droplet is ejected or not, viz., a print dot is put in an on or off state, is merely carried out in the ink jet printer. The expression of the halftone, e.g., gray, is impossible only using such a function.
There is an approach to expressing the halftone with one pixel comprising a dot matrix, e.g., a 4.times.4 or 8.times.8 dot matrix. The 4.times.4 dot matrix can express a density or optical intensity of an image using 16 levels on the gray scale (17 levels if pure white is included). If the resolution of pixels is increased, the tone is more finely expressed. If the number of gray scale levels is increased while the dot diameter is left unchanged, the resolution is substantially reduced. If the dot diameter is large on the paper, the grains are conspicuous in a region of low optical intensities. For this reason, it is necessary to reduce the weight of the ink droplet and the dot size.
There is known an ink ejecting method in which a pressure generating chamber containing ink is expanded, and then compressed to eject ink droplets (as described in Japanese Patent Application Laid-open No. Sho. 55-17589). This method is called a "draw-and-eject" method. The method enables the weight of ink droplet and the dot diameter to be reduced.
Where the dot diameter is reduced, the grains are not conspicuous in the low optical intensity region, viz., the print quality is improved. In this case, the printing speed is low, however. If the dot diameter, is halved, the printing time is increased to be four times as long as that of the printer using the usual dot diameter. The problem of the printing time elongation can theoretically be solved by increasing the drive frequency for droplet ejection or the number of nozzles four times.
A technique to realize a tone storage by ejecting ink droplets of different weights from one and the same nozzle is also proposed (as disclosed in Japanese Patent Publication No. Hei. 4-15735 and U.S. Pat. No. 5,285,215). In this technique, a plural number of pulses are applied to the print head which in turn fires a plural number of fine ink droplets. Those fine ink droplets are merged into a single large ink droplet before it lands on the paper.
The technique of the publication can eject fine and large ink droplets. However, it is technically difficult to merge the ink droplets into a single ink droplet before its landing. Because of the process of merging the fine ink droplets, a range within which the dot diameter may be varied is also narrow.
Accordingly, an object of the present invention is to provide an ink jet printer capable of ejecting a plural number of ink droplets of different weights from one and the same nozzle without reducing the printing speed, and an ink jet printing method having the same function as of the ink jet printer.
Another object of the present invention is to provide an ink jet printer capable of ejecting a plural number of ink droplets during one print period, and an ink jet printing method having the same function as of the ink jet printer.
Yet another object of the present invention is to provide an ink jet printer capable of ejecting a plural number of ink droplets during one print period while reducing data transmission time, and an ink jet printing method having the same function as of the ink jet printer.
Other objects of the present invention will be apparent from the following detailed description.